The present invention relates to a digital modulator and a digital modulation method, and more particularly relates to a technique for oversampling using a CIC (cascaded integrator-COMB) filter.
Conventionally, digital modulators have been used as transmitters or the like for digital cable TV set-top boxes and digital satellite broadcasting. In a digital modulator, after mapping, band limiting by an FIR filter is performed and then data is oversampled. Thereafter, the data is interpolated to a sampling frequency of a quadrature modulator and then is quadrature-modulated. (see, for example, Japanese Laid-Open Publication No. 2002-335293).
In another case, after mapping, band limiting is performed by an FIR filter and then data is oversampled by a CIC filter. Thereafter, the data is interpolated to a sampling frequency of a quadrature modulator and then is quadrature-modulated.